Retroviral Diseases

Overview of Retroviruses

• Retroviruses are generally benign.

• Most replication-competent retroviruses are not cytopathic.

• After chronic infection establishment, minimal cellular metabolism is dedicated to virus expression.

Classes of Pathogenicity

1. Slow leukemia viruses:

   • Examples: MuLV (Murine leukemia virus), ALV (Avian leukosis virus).

   • Each infection event relates to a proviral insertion, causing mutations.

   • Chronic viremia leads to multiple insertion events; each one poses a risk of affecting cell division control and causing abnormal cellular proliferation.

2. Acute transforming viruses:

   • Rapid tumor development.

   • Retroviral replication allows recombination between viral and host sequences, leading to inappropriate gene expression.

   • Expression of host genes at altered levels or in inappropriate cells can drastically change the cell's physiology.

   • Result: If the expressed gene is mitogenic (promoting cell division) or anti-apoptotic, it increases the likelihood of tumor formation.

3. Cytopathic retroviruses:

   • Directly kill infected cells and destroy infected tissues.

   • Example: HIV-1.

Slow Leukemia Viruses

• Cause leukemia or lymphoma after long latency periods.

• Initial lymphoid hyperplasia is observed.

• Some expanding cell subsets may progress to frank leukemia.

• Leukemogenesis (tumor formation): a multi-step process likely involving viruses at various stages.

• Proviral insertional mutagenesis:

  • Most common mechanism by which replication-competent viruses initiate tumors.

  • Provirus integrates near a growth-controlling gene, altering its expression, leading to clonal tumor development with integration at a shared site.

Mechanisms of Insertional Mutagenesis

• Insertion in the same transcriptional orientation of a provirus, within the first intron, leads to new mRNA initiated in the 3′ LTR (Long Terminal Repeat).

• Insertions upstream of genes increase expression driven by natural promoters.

• Insertion in the same orientation generates RNA that begins in the 5′ LTR and includes downstream exons.

• Insertion may lead to premature mRNA termination, generating inactive fragments.

• Insertional activation of proto-oncogenes by provirus alone is insufficient for complete transformation of cells, indicating reliance on additional mutations for tumor progression.

• Viral LTR (Long Terminal Repeat) contains essential determinants for leukemogenicity and cell tropism regarding transformation; viral capability for cell transformation is highest in cells with the most active LTR.

Acute Transforming Retroviruses

• Involve the transduction of cellular oncogenes.

• Acquire segments of cellular genes responsible for transforming activity.

• Rous Sarcoma Virus (RSV):

  • Prototype; carries a transformed form of the c-src gene.

  • Maintains all replication functions (uncommon); other viruses often lose portions of their genome, becoming replication defective but keep necessary cis-acting elements for replication via collaboration with replication-competent helper viruses.

Gene Acquisition

• Occurs through recombination events.

• Resulting genome includes only mRNA sequences (exons) from the host gene, excluding introns.

• Genes frequently acquired include:

  • Growth factors

  • Growth factor receptors

  • Intracellular tyrosine kinases

  • Transcription factors

• Often the acquired genes are fused to Gag, Pol, or Env sequences.

Examples of Acute Transforming Retroviruses

Cytopathic Retroviruses

• Example: HIV infection.

• Cell killing primarily affects CD4 T cells.

• Spike in HIV gene expression linked to cytopathicity, though not conclusively tied to specific gene products.

• HIV-1 infection results in CD4-positive cell depletion, leading to immunodeficiency and severe opportunistic infections.

HIV Genome Organization

• Order:

  • MULV LTR, gag, pol, LTR, env for standard retroviruses.

  • HTLV TR gag env pro pol rev LTR tax-tat for complex retroviruses.

  • HIV-1 LTR gag vif LTR pol-rev neti vpr env vpu-tat.

  • HIV-2 LTR gag vif LTR pol rev nef env vpx vpr.

• Total length approximates 10 kb.

• HIV genome encodes several additional protein products through alternative splicing.

HIV LTR Functions

• Primary function: Regulatory control of viral RNA synthesis.

• Initial HIV promoter recognition needs numerous general transcription factors.

• LTR comprises unique DNA elements for transcriptional regulatory factor binding.

  • Sp1: Basal level of HIV transcripts.

  • Two tandem sites for NF-kB/Rel transcription factors function as activatable enhancers for HIV-1 LTR-directed expression.

• NF-kB activation leads to enhanced virus replication, especially post T-cell activation due to various signaling pathways.

Clinical Progress of HIV Infection

CD4+ T Lymphocyte Count Overview

• Measured in cells/mm³ with a reference to the progression of HIV infection over time, observing a trajectory from 1,200 cells/mm³ at primary infection to a steep decline leading to severe immunodeficiency.

• Graph Data:

  • Decline in T-cell numbers indicates disease progression through stages of acute HIV syndrome, clinical latency, and development of opportunistic infections.

Importance of CD4+ T-cell Count

• Serves as an excellent marker for immunodeficiency degree, influencing the risk for infections and AIDS-related complications.

• Viral load predicts disease progression, effectively predicting the CD4+ T-cell count trajectory based on viral levels.

• Risk of developing AIDS within 3 years increases significantly with viral load increments, as shown:

  • CD4+ Count 351-500/μL:

  • Risk of developing AIDS:<3000 copies/mL: virtually 0%

  • 3000-10,000 copies/mL: 8%

  • 10,000-30,000 copies/mL: 16%

  • >30,000 copies/mL: 43%

• These findings are crucial for tailoring therapeutic options.

Initial Infection Phase

Primary Infection (Acute Infection)

• Often unrecognized and resembles flu-like or mononucleosis-like symptoms.

• Plasma HIV RNA levels can range from thousands to millions of copies/mL, with acute viremia resulting in a significant drop in CD4 cell numbers.

Early Stage Disease

• Approximately 10 years of clinical latency between primary infection and advanced immunodeficiency symptoms.

• Symptoms indicating depressed immune response often manifest when CD4+ T-cell counts fall below 500 cells/μL, including:

  • Oropharyngeal candidiasis (oral thrush)

  • Recurrent vulvovaginal candidiasis

  • Multidermatomal herpes zoster (shingles)

  • Oral hairy leukoplakia associated with Epstein-Barr Virus (EBV)

  • Cervical dysplasia linked to human papillomavirus infection.

Oral Hairy Leukoplakia

• Characteristic EBV replicative lesion found on the lateral borders of the tongue.

  • Notable as the first recognized oral abnormality in HIV patients.

  • The lesion shows thickened epithelium and distorted keratinocyte differentiation.

  • Typically painless and serves as a sign of AIDS.

Late Stage Disease

• Occurs when CD4+ T-cell count drops below 200 cells/μL.

• Results in susceptibility to opportunistic infections from ordinarily non-virulent organisms (e.g., Pneumocystis carinii, Mycobacterium avium).

• Increased incidence of EBV-positive malignancies such as leiosarcoma, leiomyosarcoma, and Hodgkin's disease, as well as Kaposi’s sarcoma.

Kaposi’s Sarcoma

• Associated with Herpesvirus (HHV8).

• Originates from cells lining blood vessels and lymphatic system.

• Can disseminate to organs such as the digestive tract or lungs.

Determinants of Disease Progression Rate

Host Factors

• Host genetic factors prominently affect HIV progression, notably by influencing HLA haplotypes.

• Genetic polymorphisms: Individuals homozygous for a 32-base pair inactivating mutation in the CCR5 gene (CCR5-D32) experience significant protection against HIV infection.

• CCR5-D32 heterozygotes may receive a degree of defense against disease progression.

Anti-Retroviral Therapy Overview

Treatment Modalities

Protease Inhibitors:

• HIV-1 protease cleaves Gag precursor polyprotein into essential virion components (p24, p17).

• Developed based on the protease's crystal structure to ensure effective inhibition.

• Combination strategies represent the most potent formulation by pairing RT inhibitors, which prevent new infections, with protease inhibitors, which facilitate the release of non-infectious virions from already infected cells.

• Reverse Transcriptase (RT) Inhibitors:

  • First line of treatment developed for HIV infections.

  • Most RT inhibitors are nucleoside analogs.

  • They do not act on cells already infected by HIV that no longer utilize RT for replication.

Recent Developments in HIV Management

• Truvada: Approved antiretroviral cocktail used prophylactically for individuals at high risk for HIV infection.

• A new cytotoxic T-cell population can eliminate virus-infected cells, including those infected by EBV, potentially leading to innovative therapeutic strategies by activating and expanding these cells to target the latent reservoir.